The simplest explanation is just anxiety that your brain translates into an image of losing teeth. These dreams really do happen a lot to people during stressful periods, but not because teeth themselves symbolize anything. It's more like a universal image of helplessness and loss of control that kicks in exactly when you're tense. Plus there's a physical component - during stress, people actually clench their jaws, grind their teeth at night, and your brain picks up on that.
But the previous answer about grinding isn't quite complete. Often it's not just about physical sensations, but about the fact that your body is literally in threat mode. In that state, your brain generates anxious images - loss, breakdown, destruction. Teeth are a convenient symbol here also because it's a visible part of your body that you can't hide. Subconsciously it's connected to vulnerability.
As for what to do about it - basically, deal with the stress. Learn to notice it throughout the day, add some physical activity, see if something simple like meditation or just evening walks might help. And if you notice you're grinding your teeth at night - try relaxing the muscles in your face and jaw before bed. Once the stress goes down, the dreams will go away on their own. Dream interpretation guides really won't help here - you need to work with the source, not the image.